The present invention is directed to a highly reliable fiber optic repeater, and more particularly to a repeater for use in a multi-star T-connection network. The repeater is particularly useful in industrial control and sensing applications.
Continuing improvements in the transmission quality of optical fibers, and in particular increased bandwidth and reduced attenuation rates, have made optical fiber communication networks an increasingly attractive alternative to networks which employ conductors as the transmission medium. Moreover, optical networks are intrinsically insensitive to electromagnetic noise. In order to communicate optically, an electrical signal developed within a terminal device such as, for example, a telephone, computer, or numerically controlled machine tool, is delivered to an optical transmitter within the terminal device. The optical transmitter uses the electrical signal to modulate light from a source such as an LED or laser diode. Assuming that the electrical signal developed within the terminal device is a digital signal in serial form, the modulation is typically conducted by using the electrical signal to flash the LED or laser diode ON or OFF, thereby generating the optical equivalent of the electrical signal. The modulated light is transmitted via an optical fiber to an optical receiver within another terminal device. The optical receiver includes an optical detector, such as a photodiode, which re-converts the modulated optical signal into an electrical signal. Thus the optical transmitters and optical receivers within the terminal devices, together with the optical fibers connecting them, effectively replace conductors which might otherwise have been used. A communication protocol is typically used to limit access to the network so that only one terminal device at a time can transmit. Such protocols (e.g., token passing) are known in the art and may be implemented within the electronics of the terminal devices.
A fiber-optics star is a passive hub used for collecting optical signals from a number of input fibers or for distributing optical signals to a number of output fibers. Both transmissive and reflective stars are known. The physical structure of a transmissive star is illustrated schematically in FIG. 1, wherein four optical fibers had been fused at a tapered region 20 to provide a star 22 having first ports 24, 26, 26, and 30, and second ports 32, 34, 36, and 38. Light entering star 22 through any of the first ports 24-30 is equally distributed to all of the second ports 32-38. For example, if light having an intensity of one unit were introduced into first port 24, light having an intensity of a quarter unit (neglecting minor losses) would be emitted through each of second ports 32-38. Star 22 is bi-directional; for example, light entering second port 36 would be equally distributed to first ports 24-30. Despite its bi-directional nature it is frequently convenient to refer to a star's light "input" and light "output" fibers, which is determined by how the star is oriented when it is installed for use.
Star 22 could be used to interconnect four terminal devices, each terminal device being separately connected via optical fibers to one of the first ports 24-30 and to one of the second ports 32-38. In this configuration star 22 is appropriately deemed a "mixing" star since it serves to collect optical signals from all of the terminal devices and distribute optical signals to all of the terminal devices. Mixing stars are not limited to four pairs of ports, as in the example of FIG. 1, but instead typically service from 16 to 64 terminal devices. Moreover, the basic structure of FIG. 1 could be modified to provide a collection star or a distribution star. To provide a 4-to-1 "collection" star, all of the first ports and one of the second ports, or vice versa, would be used. For example, each of the first ports 24-30 would be connected to respective sources of optical signals and one of the second ports, for example port 32, would be used to convey the collected signals, with the remaining second ports, 34-38, being terminated in a non-reflective manner. The same structure could be used in reverse, with port 32 being coupled to a signal source, as a 1-to-4 "distribution" star. Small collection or distribution stars are also known as "optical couplers."
A combination of passive stars and active repeaters is required in forming large fiber-optic networks. Since each repeater services one or more stars, the failure of a repeater--even if provisions are made for passively bypassing it--essentially disconnects one or more stars and the terminal devices connected thereto from the network. This represents an unacceptable communication loss in an industrial environment. Consequently, very high repeater reliability is essential.